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Plastico

PLC5/15 Changes its address on power-up

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We have an old machine that has a PLC 5/15 (and PLC5 hardware) which is connected to a PV550 HMI via DH+.  The PLC normally has an address of 16 and the HMI is set to 17.  On occasion, when the machine is completely powered down (for a little while) and then powered up, once the HMI is fully powered up it says that there is a duplicate node address (it worked fine before power down).  It turns out that the PLC has for some reason changed its address to 17 (the HMI's address).  I currently don't have access to the PLC (no programming software :-( ) but if I changed the HMI address (which was 17) to 16, I lose the error message (but obviously the HMI won't correctly display data).  I have to go into the HMI's program (I have that software), change the terminal node address to 16 and the PLC node address to 17, save it and download it to the HMI.  Then it works ok until the next time.

It doesn't always do this.  Quick power offs don't seem to affect it.  Only longer down times (and not always).

Any suggestions as to why this happens and how I can fix it (without going into the PLC - as I mentioned, no software and the company doesn't want to shell out $9000 for it).

Thanks,

Andrew

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The PLC5s I've worked with used DIP switches on the processor to set the DH+/RIO address; it wasn't done in software.  I can't think of any way for that address to change on power cycle.   If it only checks the DIP switch on power-up, you may have a flaky DIP switch.  In that case, I can see weird things happening sometimes.

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DH+ is a solid communication backbone.  The PLC-5/15 processor may be throwing pebbles at your window, which could become a brick soon (i.e., preeminent failure may be around the corner).  You have no programming interface and the processor may be showing signs of failure.

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If this is a critical machine (aren't they all?) I would point out what happens to production when that processor fails and you don't have a backup or any way to program a backup.  Compare the cost of extended downtime to the cost of the software or a PLC upgrade. 

For the fun of it, I just ran pricing for a CompactLogix L33ER chassis with 4 AC input modules and 4 AC output modules.  The hardware lists for just over $7000, the programming software lists for $3600.  You would have to do "something" with the HMI as well. A 6" PV+7 standard lists for about $1300 and talks Ethernet.  Software lists for just under $900.

Something to consider....

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Try changing the PLC DH+ address to 24. That way, none of the switches used to set the address to 16 will come into play if the problem is a failing dip switch.

You'll have to change your PV project to see the PLC @ Node 24.

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Smart, McNasty !   My guess is that there's condensation or corrosion on the first DIP switch, causing a short that makes the binary number go up by 1 when the PLC-5 restarts.   The apparent time-of-shutdown difference might have to do with the temperature of the enclosure. 

If the machine owner can't shell out for PLC-5 software, they should at least pay a local integrator who does have the tools to come in and do some disaster-recovery backup.   They can then run the control system to failure if they want.

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Would condensation in the first DIP switch that causes the address to go from 16 to 17 not also cause it to go from 24 to 25?

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Yes Joe, it could. Moving the PLC to 24 would prevent the duplicate node address from occurring if such is the case but as you're probably also thinking, it could be just the thing for proving that sw-1 on that DIP switch assembly is intermittent.

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On 2/2/2017 at 2:25 PM, Ken Roach said:

Smart, McNasty !   My guess is that there's condensation or corrosion on the first DIP switch, causing a short that makes the binary number go up by 1 when the PLC-5 restarts.   The apparent time-of-shutdown difference might have to do with the temperature of the enclosure. 

If the machine owner can't shell out for PLC-5 software, they should at least pay a local integrator who does have the tools to come in and do some disaster-recovery backup.   They can then run the control system to failure if they want.

Firstly, thanks to everyone who's responded.

Ken, I'll give this a try next time the machine gets shut down and see what happens.  Although the machine and PLC/IO Cards are old, the rack is only a couple of years old (had to replace it because of a bad slot).  That's not to say that the DIP Switches aren't bad or maybe a heat related issue. 

I've talked to management about this and have given my recommendations, so it's in their hands...  we do have a backup of the PLC program and replacement PLC5/15 should this die, but will obviously have to get someone in to program it (unless they finally see the wisdom in getting me the software).

Thanks everyone.

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Ah, yes, it would indeed be a good diagnostic check, even though it wouldn't fix the problem since the HMI still wouldn't communicate (it would be looking for node 24 not node 25).

 

Plastico:  the DIP switches are on the processor, not the chassis.  If you can get someone in to download the program to your spare processor (I would strongly recommend it), you can swap in the processor as a test as well.  And you can then keep it powered up in a spare chassis to keep the program in it as a spare as well.

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